Transmission line structure



June 30, 1942. H. SALINGER- TRANSMISSION LINE STRUCTURE Filed Jan. 10,1941 I Sheets-Sheet 1 INPUT OUTPUT mvtmon 'SALINGER TRANSMISSION LINESTRUCTURE Filed Jan. 10, 1941 2 Sheets-Sheet 2 FlG.3 I

INPUT OUTPUT 'NPUT OUTPUT I Th4 INVENTOR SALINGER surface.

Patented June 30, 1942 I 2,288,030 TRANSMISSION LINE STRUCTURE.

Hans Salinger, Fort Wayne, Ind., assignor to Farnsworth Television andRadio Corporation, a corporation of Delaware Application January 10,1941, Serial No. 373,869

3 Claims.

This invention relates to transmission-line structures including aplurality of transmissionline elements and particularly relates to ashielding means for such transmission-line structures.

In certain transmission-line structures including a plurality oftransmission-line elements having inner conductors and shield conductorswhich represent reactance networks, it often becomes necessary so toconnect two conductors that current fiows along the inner surface of oneof the conductors and also incidentally along its outer The currentflowing along the outer surface of such a conductor can readily fiowthrough distributed capacitance to adjacent filter elements or toground, thereby constituting a radiation loss as well as changing thetransmission characteristics of the filter. Such a condition, however,is most unsatisfactory.

Certain shield structures have been proposed for the prevention ofradiation losses. Such structures, however, are designed for onepredetermined frequency only, are ineffective at other frequencies, andare therefore not desirable for transmission of wide frequency bands.

The object of the present invention, therefore, is to provide a new andimproved transmissionline structure in which the current incidentallyfiowing along the outer surface of a conductor is returned to thetransmission-line structure and radiation loss and other detrimentaleffects avoided independently of frequency;

In accordance with the present invention there is provided atransmission-line structure including a plurality of transmission-lineelements having inner conductors and shield conductors connected so asefiectively to constitute a reactance network. The transmission-linestructure includes a connection between two of its conductors wherebycurrent fiows along the inner surface of one of the conductors and alsoincidentally along its outer surface. There is also provided a shieldconductor disposed adjacent the last-mentioned conductor and connectedthereto at one of its ends, while its other end is connected to anotherconductor of the transmission-line structure, thereby providing a returnflow of the incidental current to the transmission-line structure.

For a better understanding of the invention, together with other andfurther objects thereof,

reference is had to the following descriptiontaken in connection withthe acccompanying drawings and its scope will be pointed out in theappended claims.

In the accompanying drawings:

Fig. 1 shows a circuit diagram of a lattice-type filter, while Fig. 2schematically shows a transmission-line structure constituting such afilter. Fig. 3 schematically shows another transmissionline structureembodying the present invention and representinga bridged-Tfilter-section, while Fig. 4 shows a diagram of its equivalent circuit.Referring now more particularly to Fig. 1 of the drawings there is showna schematic diagram of a lattice-type filter having a pair ofinputterminals I, 2 and a pair of output terminals I4; I5. The inputterminals I, 2 are connected to points I3 and II, respectively, of thefilter between which the series combinations of reactances Z1 and Z2,and Z3 and Z4, respectively, are connected, as shown. The junction pointI6 between reactances Z and Z4 and the junction point il betweenreactances Z1 and Z: are connected to the input terminals I4 and I5,respectively, as is conventional.

Fig. 2 shows a transmission-line structure constructed of coaxialtransmission-line elements and constituting a lattice-type filter asshown in Fig. 1. Corresponding elements and points are indicated by thesame reference numerals in the two figures. Corresponding to the inputterminals I, 2 of Fig. 1 there is provided an input terminal line havingan inner conductor I and a shield conductor 2 which form the "input linefeeding the transmission-line structure. For the purpose of providingthe reactances Z1 and Z2 between points I3 and II there is provided atransmission-line means 3 having an inner conductor 4, a first shieldconductor 5 of predetermined diameter, and a second shield conductor 6of greater predetermined diameter, all arranged concentrically withrespect to each other. For the purpose of providing reactancesZa and Z4between points I3 and I I there is provided a second transmission-linemeans 1 comprising an inner conductor 8, a first shield conductor 9 ofpredetermined diameter, and a second shield conductor III of greaterpredetermined diameter, also concentrically arranged with respect toeach other. The shield conductor 2, as well as second shield conductors6 and I0, are all joined together to form a unitarystructure and theirjunction is generally indicated at I I which represents a substantiallyequipotential area since this area is small as compared with the actuallength of the transmission-line elements. The inner conductors I, 4 and8 are connected together at a point I3.

For the purpose of providing the structure with a signal output meansthere is provided an 2 equal to any predetermined values obtain thedesired filter action.

It will be noted that in such a structure current flowing from the pointIi to the shield'conductor I will flow along the inner surface ofconductor I5 as well as incidentally also along its outer surface. This,however, as was mentioned before, is an undesirable condition. For thepurpose, therefore, of returning the current incidentally flowing fromthe point It along the Z4 and Z5 is effective between points II and I6,

the transmission-line structure must be so de- .signed that theresultant reactance is of suitable value to render the desired filtercharacteristic.

Fig. 3 shows a bridged T-fllter-s'ection comprising coaxialtransmission-lineelements and of a general structure which is identicalwith that shown in Fig. 1 with the exception thatcertainelectricalconnections within the filter structure aredifferent,,so as to obtain the effect of a bridged T-fllter-section.points are indicated by the same reference numerals as in Fig. 1. Incontrast to the struc- 'tureof Fig. l the point I6'is connected to the'inner conductor 4, while the inner conductor I4 is connected to thejunction point. I3 and the point I1 is connected to the shield conductorI5.

Fig. 4 shows the diagram of thecircuit electrically equivalent to thestructure of Fig. 3.

Again in Fig. 4 points corresponding to those of Fig. 3 are marked bythe same numerals and the equivalents to the reactances represented bythe transmission-line elements of Fig. 3 are indicated by the labelledblocks.-

F'lg. 4 shows that the equivalent reactance elements Z5, 6 and Zn, 19,the latter including the shield I8, are connected in parallel relationwith each other. By properly designing the transmission-line elementsand theshield I8 the resultant reactance of Z5, 0 and Z13, m can readilybe made to have the proper value desired in the filter secon. I

While the structures of Figs. 1 and 3 have been marked with input andoutput ends, it will be understood that the input and output ends mayalso be interchanged as is well known to those skilled in the art.

In the above there have been shown and described transmission-linestructures provided with a shield for prevention of radiation losses andother detrimental effects which is so connected in thesetransmission-line structures, that the reactance constituted by theshield and another part of the transmission-line structure be- I comes auseful reactance element of the'network represented by transmission-linestructure.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is, therefore, aimedin the appended claims to cover all such changes and Like parts and Ynecessary to modifications as fall within the true spirit and scope ofthe invention. I

What is claimed is:

1. A transmission-line structure including a single input terminal lineand a single output terminal line, each having an inner conductor and anouter conductor, a plurality of transmission-line elements having innerconductors and shield conductors connected so as effectively toconstitute a reactance network between said input and said output lines,a connection between one of said transmission line element conductorsand the outer conductor. of one of said terminal lines whereby currentflows along the inner surface of said last-named outer conductor andalso incidentally along its outer surface, and a shield conductordisposed adjacent said lastnamed outer conductor and connected theretoat one ofits ends and to another conductor of said structure at theother of its ends, thereby to provide a return flow of said incidentalcurrent to said structure, the reactance of the transmission lineelement formed by said last-named shield conductor and said last-namedouter conductor being electrically connected in parallel relation to thereactance of one of said transmission line elements.

2. A transmission-line structure including a single input terminallineand a single output terminal line, each having an inner conductor and anouter conductor, a plurality of, transmission-line elements having innerconductors and shield conductors so as effectively to constitute areactance network between said input and said output lines, a connectionbetween one of said transmission line element conductors and the outerconductor of one of said terminal lines whereby current flows along theinner surface of said last-named outer conductor and also incidentallyalong its outer surface, and a cylindrical shield conductorconcentrically disposed about said last-named outer conductor andconnected thereto at one of its ends and to another conductor of saidstructure at the other of its ends, thereby to provide a return flow ofsaid incidental current to said structure, the reactance of thetransmission line element formed by said last-named shield conductor andsaid lastnamed outer conductor being electrically connected in parallelrelation to the reactance of one of said transmission line elements.

3. A transmission-line structure comprising a single input lineincluding an inner conductor and a shield conductor, transmission-linemeans providing a reactance networkand including an inner conductor, afirst shield conductor of predetermined-diameter, and a second shieldconductor of greater predetermined diameter, said input'line shieldconductor and said second shield conductor being joined to form aunitary outer shield structure,; a single output line including an innerconductor and a shield conductor, means for connecting said first shieldconductor of said transmission-line means to said last-mentioned shieldconductor, and shielding means concentrically arranged about said outputline shield conductor and joined at its opposite ends to said unitarystructure and said last-mentioned shield conductor, the reactance of thetransmission line element formed by said shielding means and said outputline shield conductor being electrically connected in parallel relationto oneof the reactances of said reactance network.

HANS SALJNGER.

